WO 99/47260 (the entire contents of which is incorporated herein by reference) discloses an apparatus for coating a substrate monolith comprising a plurality of substantially parallel channels extending from a first end face to a second end face thereof, which apparatus comprising means for dosing a pre-determined quantity of a liquid component, such quantity being such that may be substantially wholly retained within a substrate monolith to be coated, liquid component containment means locatable on the top of the substrate monolith to receive said quantity of liquid component, and pressure means such as over-pressure or a vacuum for transferring the liquid component from the containment means into at least a portion of the substrate monolith. An important characteristic of the disclosed apparatus and method is that substantially all of the liquid is retained within the substrate monolith, ensuring an even distribution in the desired pattern, and ensuring that there is no significant excess liquid requiring recycling. This feature is particularly important to reduce or prevent losses of the liquid component, and any precious metals contained therein. In a preferred embodiment wherein substantially all of the liquid component is retained within the substrate monolith, the method may be used to part-coat the substrate monolith in a first axial direction from one end face, before the part-coated substrate monolith is turned and the remainder of the substrate monolith is coated from the opposite end face in the same axial direction.
The description mentions that for certain coating components, the rheology of the liquid component may be such that it does not begin to enter the cells of the substrate monolith until pressure or vacuum is applied. In particular, the apparatus and method of the invention may provide plug flow of the dispensed quantity of liquid into the support.
“Rheology” may be defined as a material's response to a stress applied on the material during production, processing and/or application. Rheology measures the change in viscosity of a substance under changing shear rates, or a change in viscosity over time, at a constant shear rate.